1. The Field of the Invention
The present invention generally relates to a safety device used in motor vehicles, and more specifically, to a multi-stage inflator used to inflate an air bag during a collision or rapid deceleration.
2. The Relevant Technology
With the advances in technology, it has become well known to protect a vehicle's occupant using an inflatable device, such as an air bag or cushion. When the vehicle, usually an automobile, is involved in a collision, a crash signal actuates the inflator to cause an air bag to deploy and rapidly inflate as inflation fluids escape from an inflator to fill the air bag. The inflation fluids inflate the air bag from an uninflated condition to an inflated condition in a matter of a few milliseconds. The air bag extends into the vehicle occupant compartment and allows a reasonable deceleration of the occupant while preventing the occupant from impacting, by way of example, the steering wheel or passenger side dashboard.
The configuration of the air bag and inflator is important since the air bag and inflator must be mounted within the interior of the vehicle. Consequently, the industry is constantly striving to reduce the size of the air bag and inflator so that a smaller installation area in the vehicle is required. Additionally, the industry strives to provide a rapid expansion of the air bag, in a controlled manner, so that the occupant of the vehicle may be brought to rest faster during the collision. By achieving this, the vehicle occupants encounter lower gravitational-loads during the collision and a reduced chance of injury.
Traditional inflators utilize a single igniter assembly to generate the inflation fluid required to fill the air bag. Typically, the igniter assembly triggers a chemical, pyrotechnic, or some other reaction in the inflator. The inflator then emits the inflation fluid, usually in the form of an inflation gas, that is directed to flow into the air bag. One such device is a pyrotechnic inflator. A pyrotechnic inflator utilizes a combustible gas generating material to create the inflation fluids used to inflate the air bag. The combustible gas generating material produces inflation gases with high temperatures. Unfortunately inflators with only one igniter assembly, also known as single stage inflators, are not capable of varying the degree to which the air bag is inflated in response to the severity of the collision, nor do they reduce the gravitational-loads which a vehicle occupant endures during a collision.
One type of inflator, known as an adaptive multi-stage inflator, has been developed to address these problems. A multi-stage inflator is one which incorporates multiple igniter assemblies to generate additional quantities of inflation fluids to aid in the expansion of the air bag. An adaptive multi-stage action inflator has the additional capability to modify its performance based on various criteria such as, by way of example and not limitation, ambient temperature, severity of the impact, or position of the passengers. For example, in the event of a low speed collision a small amount of inflation fluid is released into the air bag. In contrast, in a severe collision a large amount of inflation fluid is directed into the air bag to increase the ability of the air bag to restrain and cushion the vehicle occupant.
Unfortunately, current multi-stage inflators and adaptive multi-stage inflators still have significant problems. On such problem is known as sympathetic ignition. Sympathetic ignition occurs when the heat generated by the first stage igniter assembly or the combustion of the gas generating material causes the second stage igniter assembly to unexpectedly self-activate after a given time (generally 1-4 minutes), but before the secondary igniter assembly is scheduled to be actuated. Sympathetic ignition usually occurs when the temperature at which the igniter material contained in the secondary igniter assembly will self-ignite is lower than the temperature of the first stage inflator assembly after deployment or the temperature during combustion of the gas generating material. This property is often referred to as the "auto-ignition temperature" of a material. Current non-azide generants and/or igniter materials that have been used in the secondary igniter assemblies have tended to have low auto-ignition temperatures which have contributed to this problem. Sympathetic ignition can be dangerous because if the secondary igniter assembly is not deployed in a timely manner it could potentially pressurize the air bag again and startle or possibly injure the occupant or rescue personnel after a crash.
Another problem with existing adaptive multi-stage inflators and multi-stage inflators are that they tend to be larger than single stage inflators. These type of inflators are larger because they require additional igniters and quantities of igniter material. This is problematic because of the drive to reduce the size of air bag systems in general.
A further problem with existing adaptive multi-stage inflators and multi-stage inflators is the way they direct the additional inflation fluid to the air bag. Previous adaptive multi-stage inflators and multi-stage inflators are typically configured to filter all of the inflation fluid that is directed into the air bag. The filtering process, however, reduces the temperature and pressure of the inflation fluid inside the air bag. As a result, while protecting the air bag, the filtering process reduces the effectiveness of the additional igniter assembly. Consequently, multi-stage inflators typically require a larger amount of igniter material to provide the desired pressure of the inflation gas inside the air bag.